Obligate symbiont involved in pest status of host insect

Obligate symbiont involved in pest status of host insect 

Takahiro Hosokawa, Yoshitomo Kikuchi, Masakazu Shimada and Takema Fukatsu 

Proc. R. Soc. B 2007 274, 

1979-1984 

doi: 10.1098/rspb.2007.0620 

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Obligate symbiont involved in pest status 

of host insect 

Takahiro Hosokawa 1,2 , Yoshitomo Kikuchi 1,3 , Masakazu Shimada 2 

and Takema Fukatsu 1,2, * 

1 Institute for Biological Resources and Functions, National Institute of Advanced Industrial Science and Technology (AIST), 

Tsukuba 305-8566, Japan 

2 Department of Systems Sciences, University of Tokyo, Tokyo 153-8902, Japan 

3 Department of Molecular and Cell Biology, University of Connecticut, Storrs, CT 06269-3125, USA 

The origin of specific insect genotypes that enable efficient use of agricultural plants is an important subject 

not only in applied fields like pest control and management but also in basic disciplines like evolutionary 

biology. Conventionally, it has been presupposed that such pest-related ecological traits are attributed to 

genes encoded in the insect genomes. Here, however, we report that pest status of an insect is principally 

determined by symbiont genotype rather than by insect genotype. A pest stinkbug species, Megacopta 

punctatissima, performed well on crop legumes, while a closely related non-pest species, Megacopta 

cribraria, suffered low egg hatch rate on the plants. When their obligate gut symbiotic bacteria were 

experimentally exchanged between the species, their performance on the crop legumes was, strikingly, 

completely reversed: the pest species suffered low egg hatch rate, whereas the non-pest species restored 

normal egg hatch rate and showed good performance. The low egg hatch rates were attributed to nymphal 

mortality before or upon hatching, which were associated with the symbiont from the non-pest stinkbug 

irrespective of the host insect species. Our finding sheds new light on the evolutionary origin of insect pests, 

potentially leading to novel approaches to pest control and management. 

Keywords: Megacopta punctatissima; Megacopta cribraria; Candidatus Ishikawaella capsulata; 

symbiont capsule; plant adaptation; pest evolution 

1. INTRODUCTION 

In many herbivorous insects, different populations of the 

same species often use different food plants, which are 

referred to as host races, biotypes or ecotypes. Formation 

of such intraspecific plant specialization must have evolved 

through acquisition of a new food plant by a local 

population of the insect. In the case that the new food 

plant is an agricultural plant, the insect population will be 

recognized as an emergent pest. Hence, the origin of 

specific insect genotypes that enable efficient use of 

agricultural plants is an important subject not only in 

applied fields like pest control and management but also in 

basic disciplines like evolutionary biology (Via 1990; 

Berlocher & Feder 2002; Karban & Agrawal 2002; Coyne & 

Orr 2004; Shoonhoven et al. 2005). Conventionally, it 

has been presupposed that such ecological traits are 

attributed to genes encoded in the insect genomes (Feder 

et al.1988; Hawthorne & Via 2001). 

However, recent studies have revealed that facultative 

bacterial symbionts may substantially affect various ecological 

traits of herbivorous insects. For example, several species 

of facultative symbionts play important biological roles for 

the pea aphid Acyrthosiphon pisum in specific ecological 

* Author and address for correspondence: Institute for Biological 

Resources and Functions, National Institute of Advanced Industrial 

Science and Technology (AIST), Tsukuba 305-8566, Japan 

(t-fukatsu@aist.go.jp). 

Electronic supplementary material is available at http://dx.doi.org/10. 

1098/rspb.2007.0620 or via http://www.journals.royalsoc.ac.uk. 

Received 10 May 2007 

Accepted 25 May 2007 



Proc. R. Soc. B (2007) 274, 1979–1984 

doi:10.1098/rspb.2007.0620 

Published online 13 June 2007 

contexts, including tolerance to high temperature (Montllor 

et al.2002; Russell & Moran 2006), resistance to parasitoid 

wasps (Oliver et al. 2003, 2005), resistance to pathogenic 

fungi (Scarborough et al. 2005), broadening of food plant 

range (Tsuchida et al. 2004), and others. Here, an idea arises 

that the symbiont could be involved in emergence of insect 

pest. However, although many pest insects of agricultural, 

economical and medical importance are known to be 

associated with bacterial symbionts (Bourtzis & Miller 

2003, 2006), there have been no studies demonstrating 

that the symbiont is responsible for pest status of its 

host insect. 

Many stinkbugs (Insecta: Heteroptera) are, by sucking 

plant sap and tissues, known as notorious agricultural 

pests (Schaefer & Panizzi 2000). These plant-feeding 

stinkbugs are generally in close association with gut 

symbiotic bacteria: they have a specialized midgut section 

bearing a number of caecal evaginations, in which a 

copious amount of symbiont is harboured (Buchner 

1965). When experimentally deprived of the symbiont, 

host stinkbugs suffer retarded growth and high mortality 

(Buchner 1965; Abe et al. 1995; Fukatsu & Hosokawa 

2002; Hosokawa et al. 2006). Probably the host stinkbugs 

are provided by the symbionts with essential nutrients that 

are lacking in their diet, as aphids are nutritionally 

dependent on the endocellular symbiont Buchnera aphidicola 

(Douglas 1989; Baumann et al. 2000). 

Stinkbugs of the family Plataspidae harbour an obligate 

g-proteobacterial symbiont ‘Candidatus Ishikawaella 

capsulata’ in the cavity of crypt-bearing posterior midgut 

1979 This journal is q 2007 The Royal Society

1980 T. Hosokawa et al. Symbiont-mediated pest status of host insect 

(Hosokawa et al.2006). The plataspid stinkbugs have been 

known for their unique mechanism for vertical transmission 

called ‘symbiont capsule’. When adult females lay eggs on 

their host plant, small brownish particles are always 

deposited under the egg mass. The particles encase a 

copious amount of the symbiont inside, and hatchlings from 

the eggs orally acquire the symbiont from the capsule 

(Schneider 1940; Müller 1956; Fukatsu & Hosokawa 

2002; Hosokawa et al. 2005, 2006; figure 1a; electronic 

supplementary material, movie 1). When deprived of 

the symbiont capsule, the symbiont-free insects suffer 

abnormal coloration, retarded growth, smaller body size, 

higher mortality and sterility (Müller 1956; Fukatsu & 

Hosokawa 2002; Hosokawa et al. 2006). Hence, despite 

the extracellular location in the gut cavity, the symbionts 

are regarded as obligate mutualistic associates for the host 

stinkbugs. The symbiont phylogeny shows a perfect 

agreement with the host phylogeny, indicating strict host– 

symbiont cospeciation over evolutionary time (Hosokawa 

et al. 2006). A number of plataspid species have been 

reported as pests of various legumes and other agricultural 

plants (Tomokuni 1993; Schaefer & Panizzi 2000). 

In this study, by making use of the unique transmission 

system mediated by symbiont capsules, we experimentally 

demonstrated that pest status of a plataspid stinkbug is 

determined by the symbiont genotype rather than by the 

insect genotype. 

2. MATERIAL AND METHODS 

(a) Insect sampling and rearing 

Freshly laid egg masses of Megacopta punctatissima were 

collected from Pueraria lobata at Tsukuba, Ibaraki, Japan. Egg 

masses of Megacopta cribraria were obtained from sexually 

mature females that had been collected from Pueraria 

montana at Naha, Okinawa, Japan. Each of the field-collected 

females of M. cribraria was allowed to oviposit in a Petri dish 

containing several pea pods (Pisum sativum). To avoid 

pseudoreplication, all of the egg masses of M. cribraria used 

for experiments were derived from different females. These 

insects were maintained in climatic chambers at 258C ina 

long-day regimen (16 h light–8 h dark). 

(b) Experimental manipulation 

Only newly laid egg masses containing 24 or more eggs and 7 

or more capsules were used for experiments. All of these egg 

masses exhibited normal egg hatch rates ranging from 80 to 

100%. To minimize confounding effects of the insect genetic 

backgrounds, two experimental groups were produced from 

each of the egg masses for control treatment and symbiontreplacing 

treatment, respectively. Each of the egg masses was 

separated into individual eggs and capsules by using fine 

forceps under a binocular microscope. To sterilize remnant 

symbiont cells possibly adhering to the egg surface, the eggs 

were treated with 70% ethanol for 5 min, 4% formalin for 

30 min and 70% ethanol for 10 s, and were dried in air. For 

control treatment, randomly chosen 12 eggs and 7 capsules 

from an egg mass were realigned like a natural egg mass and 

were glued on a piece of filter paper. For symbiont-replacing 

treatment, the other 12 eggs from the egg mass were realigned 

with 7 heterospecific capsules in the same way. These 

experimental egg masses were individually kept in a Petri 

dish with a wet cotton ball. 

Proc. R. Soc. B (2007) 



(c) Fitness measurement 

For each of the stinkbug species, nymphs from 18 sets 

(families) of a pair of experimental egg masses were subjected 

to fitness measurements. After a day of egg hatch, 10 nymphs 

were randomly chosen from each of the egg masses and reared 

on a potted soya bean plant (Glycine max). These insects were 

examined for growth rate (nymphal period), adult emergence 

rate (%) and adult body size (thorax width). 

Adult insects that emerged from the 17 out of 18 sets for 

M. punctatissima and those from the 16 out of 18 sets for 

M. cribraria were examined for their reproductive performance. 

Each of several (1–5) pairs of a male and a female 

from the same egg masses was separately kept in a Petri dish 

and provided with pea pods. The dishes were inspected 

everyday and the pea pods were periodically replaced with 

fresh ones. All egg masses laid by each of the females were 

collected during 30 days after the first oviposition. Each of 

the egg masses was kept in a Petri dish and inspected, and the 

eggs were categorized into hatched eggs, fertilized unhatched 

eggs and unfertilized eggs. Nymphal body segments and/or 

red eyespots were seen in fertilized eggs but not in unfertilized 

eggs. Nymphs that died during hatching were counted as 

fertilized unhatched eggs. For each of the females, total 

number of eggs, egg fertilization rate (%) and egg hatch rate 

(%) were calculated. Females that died without producing 

eggs were excluded from the analyses. Proportions of such 

females were statistically not different between the control 

and symbiont-replaced treatments (5 out of 62 control 

females versus 5 out of 58 symbiont-replaced females in 

M. punctatissima, Fisher’s exact probability test, pZ1; 4 out of 

45 control females versus 6 out of 43 symbiont-replaced females 

in M. cribraria, Fisher’s exact probability test, pZ0.517). 

(d) Cloning and sequencing 

DNA was extracted from isolated symbiont capsules by using 

a QIAamp tissue mini kit (QIAGEN), from which a 1.7 kbp 

segment of eubacterial groE gene was amplified by polymerase 

chain reaction (PCR) with primers Gro-F1 (5 0 -ATG 

GCA GCW AAA GAC GTA AAT TYG G-3 0 ) and Gro-R1 

(5 0 -TTA CAT CAT KCC GCC CAT GC-3 0 ). The PCR 

product was cloned and sequenced as described previously 

(Kikuchi & Fukatsu 2003). The groE sequences of the 

symbiont from M. punctatissima and M. cribraria were 

deposited in the DNA Data Bank of Japan database with 

accession numbers AB231904 and AB264337, respectively. 

(e) Symbiont quantification 

We constructed seven and six sets of control and symbiontreplaced 

egg masses for M. punctatissima and M. cribraria, 

respectively. After a day of egg hatch, 10 nymphs from each of 

the experimental egg masses were individually subjected to 

DNA extraction and symbiont quantification. The symbiont 

titres were measured in terms of bacterial groE gene copies by 

using a quantitative PCR technique as described previously 

(Koga et al. 2003). Since the symbiont groE gene sequences 

were completely identical between M. punctatissima and 

M. cribraria (accession nos. AB231904 and AB264337, 

respectively), the same primers MEGAgro-F1 (5 0 -GGT 

GCT GCC ACT GAA GTT GA-3 0 ) and MEGAgro-R1 

(5 0 -CCG CTA CAC GCA CTA ACG C-3 0 ) and the same 

probe MEGAgro-P1 (5 0 -TGA AGA AGG TGT CGT TCC 

CGG AGG-3 0 ) were used.

(f ) Statistics 

To statistically evaluate the differences between the control 

and the symbiont-replacing treatment, the Wilcoxon signedrank 

test was adopted for adult emergence rate, and a 

generalized linear model (GLM) framework was applied to 

the other fitness parameters (McCullagh & Nelder 1989). In 

the GLM analyses, for egg fertilization rates and egg hatch 

rates, we used binominal error or, if overdispersion was 

detected, quasi-binominal error (Crawley 1993, 2005), 

whereas for the other data, an appropriate error distribution 

was selected from normal, Poisson, gamma, inverse normal 

and negative binomial errors according to the Akaike 

information criterion. Two terms, namely symbiont types 

(control and replaced) and family, were included in the 

models, and the effects of the terms were evaluated by an 

analysis of deviance (Crawley 1993). All the statistical 

analyses were conducted by using a software R v. 2.3.1 

(R Development Core Team 2006). 

3. RESULTS AND DISCUSSION 

(a) Closely related pest and non-pest plataspid 

stinkbugs 

The plataspid stinkbug M. punctatissima (figure 1b) is 

commonly found in the mainland Japan, while a closely 

related stinkbug M. cribraria (figure 1c) is distributed across 

the southwestern islands of Japan. They are classified into 

different species morphologically: e.g. M. cribraria is smaller 

in size and paler in colour than M. punctatissima (figure 1b,c). 

However, they are no doubt close genetically and biologically: 

their mitochondrial 16S rRNA genes showed 99.3% 

(1565/1576 nucleotide sites) sequence identity (Hosokawa 

et al. 2006); reciprocal crosses between the species resulted 

in F 1 offspring with intermediate morphological traits; and 

crosses between the F 1 insects could produce F 2 offspring 

(T. Hosokawa 2005, unpublished data). The main host 

plants of M. punctatissima and M. cribraria are wild 

leguminous vines P. lobata and P. montana, respectively, 

while these insects also use other leguminous plants 

(Tomokuni 1993). In particular, M. punctatissima has been 

known as pest of soya bean, pea and other legumes. The 

insects often gregariously infest the plants and damage 

the crops, and without spraying, lay eggs and proliferate in 

the legume fields (Kono 1990; Tomokuni 1993; Endo et al. 

2002). On the other hand, M. cribraria scarcely causes such 

serious problems in Japan, although infestation on soya 

bean and other legumes has been occasionally reported 

(Tomokuni 1993). What is the basis of the difference 

between the pest and non-pest insects? 

(b) Fitness parameters of pest and non-pest 

stinkbugs on crop legumes 

In order to address the question, we evaluated the general 

performance of the pest species M. punctatissima and the 

non-pest species M. cribraria on potted soya bean plants 

and pea pods. Both species normally grew to adult and laid 

eggs (figure 2b–e). The eggs were certainly fertilized 

(figure 2f ). However, egg hatch rates were strikingly 

different between the species; around 80% in 

M. punctatissima in contrast to only 50% in M. cribraria 

(figure 2g). The difference was statistically significant 

(median test, p!0.0001). A characteristic mortality 

symptom was observed in the egg masses of M. cribraria, 

wherein many nymphs failed to escape from the eggshell 

Proc. R. Soc. B (2007) 



Symbiont-mediated pest status of host insect T. Hosokawa et al. 1981 

and died (figure 3b). These dead nymphs were generally 

frail in size and morphology, probably due to developmental 

abnormality (data not shown). 

(c) Low egg hatch rate of non-pest stinkbug 

on crop legumes 

These results indicated that the non-pest species 

M. cribraria suffers low egg hatch rate on the crop legumes 

while the pest species M. punctatissima does not, which is 

probably relevant to their different pest status. The crop 

legumes, although used under the laboratory condition, 

are unsuitable host plants for M. cribraria. 

(d) Experimental symbiont exchange between 

pest and non-pest stinkbugs 

In most of the obligate endosymbiotic systems in insects, 

such as those in aphids and tsetse flies, the host and the 

symbiont are structurally, functionally and developmentally 

integrated into an almost inseparable biological entity 

(Buchner 1965; Douglas 1989; Baumann et al. 2000; 

Braendle et al. 2003). Thus, it has been practically 

impossible to manipulate these obligate host–symbiont 

associations experimentally. However, the unique capsulemediated 

transmission system in the plataspid stinkbugs 

enabled us to attempt such experiments despite the obligate 

nature of the symbiosis. The host eggs and the symbiont 

capsules were separated by using forceps under a binocular 

microscope, and the eggs of M. punctatissima were combined 

with the capsules of M. cribraria, and vice versa. The 

hatchlings readily accepted the heterospecific capsules and 

ingested the content. Quantitative PCR assays confirmed 

that the nymphs certainly acquired the heterospecific 

symbiont cells, and the acquired amount was equivalent to 

that of the conspecific symbiont cells (figure 2a). 

(e) Fitness parameters of pest and non-pest 

stinkbugs on crop legumes after symbiont 

exchange 

The symbiont-replaced insects normally grew to adult 

and laid eggs (figure 2b–e). Although symbiont-eliminated 

adults of these stinkbugs were reported to severely suffer 

abnormal coloration and reduced body size (Fukatsu & 

Hosokawa 2002; Hosokawa et al. 2006), the symbiontreplaced 

adults were almost indistinguishable from the 

control adults (figure 1b,c), except that growth rate and 

body size of M. cribraria were slightly but significantly 

reduced in association with the symbiont replacement 

(figure 2c,d ). In both the species, the eggs laid by the 

symbiont-replaced adults were certainly fertilized 

(figure 2f ), but egg hatch rates were strikingly different 

between the species; around 90% in M. cribraria in 

contrast to only 25% in M. punctatissima (figure 2g). 

The difference was statistically significant (median test, 

p!0.0001). In the egg masses deposited by the symbiontreplaced 

females of M. punctatissima, many nymphs failed 

to escape from the eggshell and died (figure 3c), which was 

reminiscent of the symptom observed with the egg masses 

deposited by the normal females of M. cribraria (figure 3b). 

(f ) Pest status of Megacopta punctatissima 

determined by symbiont genotype 

In summary, the fitness measurements on the crop 

legumes in combination with the symbiont-replacing 

experiments demonstrated that (i) M. punctatissima that


(a) 

(b) M. punctatissima (c) 

M. punctatissima 

M. cribraria 

control replaced control replaced 

Figure 1. Pest and non-pest plataspid stinkbugs. (a) Newborn 

nymphs of Megacopta punctatissima probing capsules for 

symbiont acquisition. Arrows and arrowheads indicate 

symbiont capsules and eggshells, respectively. (b) Pest species 

M. punctatissima. (c) Non-pest species Megacopta cribraria. 

Normal adult females with their original symbiont (control) 

and manipulated adult females whose symbiont was 

experimentally replaced by the heterospecific one (replaced) 

are shown. Scale bars, 1 mm. 

originally exhibited a normal egg hatch rate suffered a low 

egg hatch rate when infected with the symbiont from 

M. cribraria, (ii) M. cribraria that originally exhibited a low 

egg hatch rate restored a normal egg hatch rate when 

infected with the symbiont from M. punctatissima, (iii) the 

mortality symptom in hatchlings was similar irrespective 

of the stinkbug species and was associated with the 

symbiont from M. cribraria, and (iv) hence, the normal 

egg hatch rate and the good performance of the stinkbugs 

on the crop legumes were attributed to the symbiont from 

M. punctatissima. These results strongly suggest that the 

pest status of M. punctatissima is principally determined by 

the symbiont genotype rather than by the insect genotype. 

(g) Evolutionary origin of pest-related symbiont 

genotype? 

The mechanism whereby the symbiont from M. punctatissima 

can support the normal development of the host 

insects on the crop legumes is unknown. In plataspid 

stinkbugs, the host insect phylogeny perfectly agreed with 

the symbiont phylogeny, indicating stable host–symbiont 

association over evolutionary time (Hosokawa et al. 2006). 

Among the plataspid stinkbugs phylogenetically analysed, 

M. punctatissima and M. cribraria are the closest 

genetically (Hosokawa et al. 2006). The symbionts from 

M. punctatissima and M. cribraria are also very close 

genetically as their host insects are: their 16S rRNA genes 

showed 99.9% (1308/1309 nucleotide sites) sequence 

identity, and their genome size was estimated to be 

0.82 Mbp in common (Hosokawa et al. 2006). It appears 

probable, although speculative, that mutations occurring 

in the symbiont genomes after the host speciation have 

modulated their capability of using different host plants, 

which predisposed the host insects to potentially become 

pests or not. 

Proc. R. Soc. B (2007) 



(a) 

(b) 

(c) 

(d) 

(e) 

( f ) 

(g) 

symbiont titer (×10 7 ) 

adult emergence (%) 

nymphal period (days) 

thorax width (mm) 

total no. of eggs 

fertilized eggs (%) 

hatched eggs (%) 

2 

1 

0 

100 

50 

0 

48 

44 

40 

36 

32 

3.9 

3.7 

3.5 

3.3 

3.1 

200 

150 

100 

50 

0 

100 

75 

50 

25 

0 

100 

75 

50 

25 

0 


70 

18 

n.s. 

57 

control 

n.s. 

70 1 60 

n.s. 

n.s. n.s. 

100 

18 50 18 18 

0 

control replaced control replaced 

p < 0.05 p < 0.0001 

86 77 

91 92 

3.0 

77 78 

92 

88 

female male 

2.8 

female male 

n.s. 

p < 0.0001 

n.s. 

3 

2 

0 

48 

44 

40 

86 79 91 93 

36 78 81 

97 

93 

female male 

32 

female male 

n.s. 

3.4 

n.s. 

n.s. 3.2 

p < 0.001 

150 

100 

M. cribraria 

n.s. 

57 53 50 

41 37 

control replaced 

0 


n.s. 100 

75 

n.s. 

57 53 50 

25 

41 37 


0 


p < 0.0001 

60 

37 

53 

41 

replaced control replaced 

Figure 2. Fitness measurements of normal and symbiontreplaced 

plataspid stinkbugs. (a) Symbiont titre acquired by 

newborn nymphs, in terms of symbiont groE gene copies per 

insect. (b) Adult emergence rate (%). (c) Growth rate, in 

terms of nymphal period (days). (d ) Adult body size, in terms 

of thorax width (mm). (e) Total number of eggs produced by 

an adult female. ( f ) Fertilization rate of eggs (%). (g) Hatch 

rate of eggs (%). Means and standard deviations are shown. 

Sample sizes are indicated on columns. Statistically significant 

differences between control treatment and symbiontreplacing 

treatment are shown in red.

control 

replaced 


(a) (b) 

(c) (d) 

M. cribraria 

Figure 3. Mortality symptom observed with hatchlings of 

plataspid stinkbugs reared on the crop legumes. (a,c) 

Megacopta punctatissima, (b,d ) M. cribraria, (a,b) egg masses 

laid by normal females and (c,d ) egg masses laid by symbiontreplaced 

females. Scale bars, 1 mm. 

(h) What underlies symbiont-mediated fitness 

effect on host insect: plant-specific difference 

or general difference in vigour? 

For herbivorous insects, being pests basically requires the 

ability to use an alternative suboptimal host plant 

(Karban & Agrawal 2002; Shoonhoven et al. 2005). For 

M. punctatissima and M. cribraria, wild leguminous vines 

Pueraria spp. are the main optimal host plants. Both 

species perform well on their original host plant, attaining 

over 90% egg hatch rates under natural conditions 

(T. Hosokawa 2005, unpublished data). Meanwhile, 

soya bean and other crop legumes are regarded as auxiliary 

suboptimal host plants for these insects. We found 

that normal M. cribraria and symbiont-replaced 

M. punctatissima suffer low egg hatch rates on the crop 

legumes (figures 2g and 3), indicating that the symbiont 

from M. cribraria is responsible for the poor performance 

of the insects on suboptimal host plants. In other words, 

the symbiont from M. punctatissima was superior to the 

symbiont from M. cribraria in supporting normal development 

of the host insects on the suboptimal host plants. 

There are two mechanisms as to how these symbionts 

differ in their effects on the host fitness. One mechanism is 

a plant-specific difference, wherein the symbiont from 

M. punctatissima physiologically performs better for the 

host insects than the symbiont from M. cribraria 

specifically on the suboptimal host plants. The other 

mechanism is a general difference in vigour, wherein the 

symbiont from M. punctatissima generally shows better 

performance for the host insects than the symbiont from 

M. cribraria, and the difference becomes obvious on the 

suboptimal host plants but not on the optimal host plants. 

If the former scenario is true, we should consider the 

possibility that the biological trait of the symbiont from 

M. punctatissima might have been selected for in the 

mainland Japan where the crop legumes are widely 

cultivated. On the other hand, if the latter scenario is 

true, the possibility becomes less likely, and the difference 

might be attributed to more general aspects of the 

symbiont genomes. To verify which of these mechanisms 

better accounts for the phenomena, further experimental 

studies, in particular fitness measurements of the 

Proc. R. Soc. B (2007) 



Symbiont-mediated pest status of host insect T. Hosokawa et al. 1983 

symbiont-manipulated insects on the optimal host plants, 

are needed. 

(i) Plant specialization of herbivorous insect 

mediated by obligate symbiont 

Recent studies have revealed that some facultative 

microbial symbionts substantially affect various ecological 

traits of herbivorous insects including plant specialization 

(Montllor et al. 2002; Oliver et al. 2003, 2005; Tsuchida 

et al. 2004; Scarborough et al. 2005; Leonardo & Mondor 

2006; Russell & Moran 2006). Our discovery indicates 

that even obligate symbionts that play essential biological 

roles for their host may also affect plant specialization, and 

suggests that such symbionts could potentially be causal 

agents of emergent insect pests. It is currently unknown 

how prevalent similar cases of symbiont-mediated plant 

specialization are in natural and agricultural ecosystems. 

In this context, it is of both evolutionary and practical 

importance to survey the correlation between symbiont 

genotypes and host races/biotypes/ecotypes in various 

insect–microbe symbiotic systems. 

(j) Perspective for pest control and management 

A number of agriculturally, economically and medically 

notorious insect pests harbour symbiotic micro-organisms 

(Bourtzis & Miller 2003, 2006). In some of these cases, 

the symbionts have been suggested as possible agents for 

controlling the pests by using paratransgenic approach 

(Durvasula et al. 1997; Ben Beard et al. 2002), symbiontdriven 

population replacement (Dobson 2003; Sinkins & 

Gould 2006) and incompatible insect technique (Zabalou 

et al. 2004). Strikingly, it was reported that the most 

widely applied biological insecticide, Bacillus thuringiensis, 

is effective to lepidopteran larvae only when the insects 

harbour a gut microbial community (Broderick et al. 

2006), illuminating profound relevance of insect gut 

bacteria to pest control. The gut symbiotic bacteria of 

the plataspid stinkbugs provide a model system for 

understanding the mechanisms underlying the symbiontmediated 

pest evolution, which would potentially lead to 

novel means of pest control and management. Functional 

and genomic analyses of the stinkbug symbiont would lead 

to further insights into how the symbionts affect such 

ecological traits of the host insects. 

We thank S. Ohno and D. Haraguchi for collecting samples, 

and E. Kasuya for statistics. This study was financially 

supported by the National Institute of Advanced Industrial 

Science and Technology (AIST) and the 21st COE Program 

‘Research Centre for Integrated Science’ (E-5) at the 

University of Tokyo. T.H. and Y.K. were supported by the 

Research Fellowship of the Japan Society for the Promotion 

of Science for Young Scientists. 

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Obligate symbiont involved in pest status of host insect

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